The present invention relates to frequency-shift keying and, in particular, to a device and a method of sequence detection of frequency-shift-keyed signals.
Frequency-shift-keying is employed in, for example, telecommunication for encoding messages and addresses. Frequency-shift keying is gaining in importance in this context.
Transmitting data by means of frequency-shift keying is performed in accordance with a modulation rule. In binary frequency-shift keying (BFSK) in accordance with known technology, for example, a sinusoidal signal of a frequency f0 is emitted by a transmitter in the case of a logic value “0”, and a sinusoidal signal of a frequency is emitted in the case of a logic value “1”. f0 here may be considered to be a first encoding frequency: f1 may be considered to be a second encoding frequency. f0 and f1 are of different values: f0≠f1. In the case of a logic value “0”, a signals0(t)=A cos(2πf0t+θ0)  (1)
is emitted by the transmitter for a bit duration Tb, with a logic value “1”, in contrast, the signals1(t)=A cos(2πf1t+θ1)  (2)
with phase positions of the two signals θ0 and θ1 which are generally different, and an amplitude A, as is exemplarily described in    [1] F. Xiong, Digital Modulation Techniques, Artech House, Inc., second edition, 2006.
The receiver may then, using the frequency, derive from the signal received whether a logic value “0” or “1” has been transmitted by the transmitter.
Spread frequency-shift keying (SFSK) receivers are a special variation of FSK receivers. For SFSK receivers, see in particular:    [2] T. Schaub, Spread Frequency Shift Keying, IEEE Transactions on Communications, February 1994, Edition 42, Number 234, pp. 1056-1064.
In conventional FSK receivers, a decision as to which logic value has been transmitted by a transmitter is made using a threshold value comparison, irrespective of whether the phase positions are known (coherent FSK receiver) or not (incoherent FSK receiver). If, in the period Tb, there is more signal energy near the frequency f0, the receiver will decide for the logic value “0”; if, however, there is more signal energy near f1, the receiver will decide for the logic value “1”.
When one out of M known sequences having a length of NS successive and FSK-modulated logic values is transmitted by the transmitter, a conventional receiver will try to determine the sequence transmitted by the transmitter by comparing the logic sequence determined from the signal received in accordance with the principle described above to all the M known sequences and selecting that sequence for which similarity to the sequence received is the greatest. This is generally done by calculating the correlation coefficients (or quantities proportional to said quantity) between the sequence received and all M known sequences. When assuming at M known sequences Ri, known to both the transmitter and the receiver, of a length NS, the decision of the receiver may, starting at the sequence S received of a length NS for one of the M reference sequences, be expressed as follows:
                              δ          =                                                    arg                ⁢                                                                  ⁢                max                            i                        ⁢                          (                                                ∑                                      j                    =                    1                                                        N                    S                                                  ⁢                                                                  ⁢                                                      R                    ij                                    ⁢                                      S                    j                                                              )                                      ,                                  ⁢                  i          =          1                ,        2        ,        …        ⁢                                  ,                  M          .                                    (        3        )            
This means that the receiver calculates, for each of the M reference sequences Ri, the correlation to the sequence S received and subsequently decides for that sequence Ri, with i=δ, from R for which the correlation value will be the greatest. R here describes a table of all M reference sequences Ri and Rj describes the j-th element of Ri. When S is the sequence transmitted by the transmitter, the receiver has made the correct decision, if Rδ=S, otherwise the receiver has made the wrong decision.
The procedure described here of conventional receivers is of particular disadvantage when the transmission characteristics of the entire system including transmitter, transmission channel and receiver do not represent a constant function of frequency.
The object underlying the present invention is providing improved concepts of sequence detection for frequency-shift keying. Both with frequency-independent and frequency-selective transmission behavior, correct decisions of sequence detection are to be made with high probability, without the obligation of the receiver having to know the transmission behavior and having to know the phase positions of the signals received.